The present invention relates to wireless communications, and more particularly to the sensing of wireless transmissions from a user of a spectral resource.
The radio spectrum is a limited resource that should be shared between many different types of equipment such as cellular, home network, broadcast, and military communication equipment. Historically, each part of the radio spectrum has been allocated to a certain use (called a “licensed” and/or “primary” use). This strategy has resulted in all applications/uses being disallowed on the allocated carrier frequency except for those applications included in the license agreement. In practice, this results in large parts of the radio spectrum being unused much of the time. For instance, in the Ultra-High Frequency (UHF) band, where TV broadcasts take place, large geographical areas are unused, mainly due to the large output power needed for such applications; this large output power compels a large reuse distance in order to minimize the risk of interference. An example of such geographical areas within Scandinavia is illustrated in FIG. 1. In FIG. 1, the shaded areas represent geographic locations in which a given carrier frequency is being used by a licensed user (e.g., by Broadcast TV). In the remaining areas, the so-called “white spaces”, the given carrier frequency is allocated to the licensed user but is not actually being used by that user.
In order to make better use of the licensed spectral resources, some countries will, in the future, allow unlicensed services (so called “secondary” uses) to take place in areas (called “white spaces”) in which the licensed (primary) user is not transmitting. However the primary user will always have priority for the use of the spectrum to the exclusion of others. Therefore, some kind of sensing mechanism is needed in the (unlicensed) devices to enable them to detect whether a licensed user is currently transmitting. If such licensed use is occurring, the unlicensed user needs to turn off its transmission on that carrier frequency. The most straightforward sensor is a signature detector adapted to detect specific signatures transmitted from the licensed/primary user (typically implemented as a matched filer).
Another consideration regarding the sensing of the licensed user's transmissions is placement of the sensors. When the secondary (e.g., unlicensed) use is for cellular telecommunications, one solution is to include the sensors in the base station of the mobile communication system. Sometimes, the base station's (or network's) own sensors do not provide enough information (e.g., information about the geographical positions of active white space transmitters) for the base stations to have a clear picture of white space spectrum availability as a function of geographical position. Without this information, it is difficult for a base station to use the white space fully. To compensate for this lack of information, it may be necessary to impose quite wide safety margins (for example with respect to frequency and/or power) in order to prevent the unlicensed user's interfering with the primary (licensed) user's use of white space frequencies.
The base station/network may use input from its sensors to estimate received powers at the sensors from a white space transmitter and, given the powers at the known positions of the sensors, infer where the white space transmitter may be located. However, if the propagation conditions of the paths to the different sensors are different and/or if there are a plurality of simultaneous white space transmitters, then such inference may end up wrong.
As an alternative, the base station/network may use input from its sensors to identify the location of white space transmitters by means of triangulation (i.e., positioning using so called multilateration). However, at least three sensors with reliable propagation paths from the white space transmitter are required for two-dimensional multilateration (two sensors give only one hyperbolic curve of possible positions). Finding three such sensors may not be a problem, but the multilateration process itself is based upon estimating time differences of arrival of the white space transmitter output to the different sensors. For a white space transmitter with unknown signal content (i.e., no known pilots or synchronization signals to use as time references), it may be required that large amounts of base band sensor data be transmitted to a common node in the network, where the estimation of the time differences of arrival may be performed (e.g., by correlation methods at a baseband level). In some situations there may even be a plurality of white space transmitters that simultaneously transmit unknown signal content at the same frequency from different locations. In such instances, multilateration (using the unknown content of the signals) at a baseband level will run into difficulties when trying to separate and correlate signals.
For the sake of clarity, the problems discussed above have been presented in the context of an unlicensed user wishing to detect the presence of a licensed user's signals. However, such problems are not limited to situations involving only licensed and unlicensed users, but can arise whenever communication equipment wishes to operate in a geographical area without disturbing transmissions from other communication equipment.
Therefore, there is a need for systems and methods that enable communication equipment to locate other transmitters operating within a given geographical area in an efficient and reliable manner.